There are a variety of head gaskets known in the prior art. Head gaskets generally comprise thin flat bodies which define a plurality of apertures, including oil and water openings and cylinder openings. These apertures are positioned in the gasket body to be aligned with cylinder bores and with water and oil ports in an engine block and engine head. The gasket body itself is intended to seal around the apertures which it defines thereby to prevent bore and port intercommunication and to prevent leakage from the bores.
In high-compression diesel engines operating at high temperatures and high pressures, it has been a practice for a number of years now to provide head gaskets with armoring adjacent the cylinder or combustion openings to seal the cylinder openings and to protect the remainder of the gasket from the effects of the high temperature and pressure. Quite frequently the gasket body itself comprises an asbestos or reinforced asbestos or impregnated asbestos packing material, which body is then provided with the armoring. A construction of that general character which is known to the art is illustrated in U.S. Pat. No. 1,819,694. In that construction, the entire gasket surface as well as the cylinder openings in the gasket is armored.
It has been found subsequently that armor enclosing the edge of the gasket body adjacent the cylinder opening is sufficient to protect the gasket and to seal the head and block at the cylinder bore. In one typical construction, also known to the art, a main gasket body comprises sheets of impregnated asbestos secured to opposite sides of a metal sheet. The combustion openings in the gasket body are enclosed by U-shaped metal annuli which surround the periphery of the combustion openings in the gaskets and which extend outwardly over and under the periphery of the opening. Although such constructions are suitable for many applications, they are not for others. In some constructions for which they are usually suitable, they do not always seal correctly and therefore sometimes permit leakage.
It has also been suggested that head gasket assemblies be provided in which an armored gasketing is positioned in a combustion opening and which is suspended from tabs which project outwardly of the combustion opening and into engagement with the gasket body. This very substantially improves upon known prior art head gaskets and an improved head gasket of this type is illustrated in U.S. Pat. No. 3,565,449.
More recently, diesel engines have been introduced into automobiles in increasing numbers. In typical diesel engines used in the United States, it has become a practice to provide a precombustion chamber for each cylinder bore which feeds fuel into the cylinder bore and to position that precombustion chamber (frequently referred to as a pre-cup) in the head and straddling the armoring. To insulate the gasket, to effect a seal, and to provide a seal for the pre-cup, a conventional diesel gasket, such as of the type shown in U.S. Pat. No. 3,565,449, has been used with an expensive tab (a pre-cup or precombustion tab) extending from the armor and outwardly over the main gasket body.
It has been found that such modified gaskets tend to permit blow-by, especially when the engine is cold, and as a result, the gaskets too frequently fail. Worse, the blow-by sometimes communicates the oil and water ports, causing blow out of the oil or water, the cooling media for the engine. If that occurs and is not quickly ascertained, the engine itself can burn out, and this has occurred with sufficient frequency to be a serious problem.
It has been determined that one of the possible reasons for the failure of the head and pre-cup to be sealed by the armoring, as a head without a pre-cup will usually do, is the discontinuity at the intersection between the head and pre-cup where it crosses or intersects the armor in the zone in which the wire of the armor underlies the armor sheath. For example, if the pre-cup projects beyond the plane of the head, then at the intersection with the wire, the wire is placed under a greater compressive load by the pre-cup than by the head. As a result, at the intersection there is a zone which may not force the ensheathing armor into sealing engagement with the head, the projecting side of the pre-cup and then the pre-cup bottom surface. The same result can occur if the pre-cup is recessed, or if there is a significant chamfer at the surface of the bore receiving the pre-cup. In that case there will be a zone in which the armor does not adequately seal the confronting head and pre-cup surfaces, and therefore there is a possible path for the escape of high presure gases from the cylinders, with the attendant damage these gases can inflict.
Of course, one solution to the problem would be to grind the faces of the pre-cup and head to a common plane and to make absolutely certain there is no chamfer at the head or pre-cup surface. However, that is impractical and expensive, and is not within the control of gasket manufacturers. Another solution would be to assemble parts so that they are absolutely flush. However, that is not a practical solution either, for tolerances of even several thousandths of an inch could not be permitted and holding to zero tolerances for pairs of mass-produced assembled parts is extremely difficult and exceedingly expensive.
Further, frequently it is not possible simply to increase the head load or to use a thicker or softer wire. Available head loads are limited and are normally used up to their maximum in engines of the type under consideration, and the load distribution in the zones of the armor and main body of the gasket have been calculated and determined. To substantially increase the portion of the load taken up in the area of the armor would reduce the load available in the main body and would permit leakage there.